1. Field of the Invention
The present invention relates to neutron radiography, and more specifically, it relates to rotating aperture systems for use in high-energy neutron imaging applications.
2. Description of Related Art
Conventional rotating aperture systems for use in neutron radiography use close tolerances so the rotor and stator are in close proximity but non-contacting during rotation. This leaves a small, but definite leakage path for gas to escape through the aperture system when it is closed.
Stopping a 7 MeV, >50 μA average current deuterium beam on a solid target requires angled or rotating target systems that are water cooled since the power density in the beam is >120 kW. This tends to put a large amount of material in the path of the neutron beam and would produce shadows on the object image and adversely impact interpretation of the image.
Stopping a deuterium beam with a solid beam stop will implant deuterium in the solid that will begin to produce background (off-energy) neutrons which will decrease the resolution and contrast in a neutron image.
The main way leakage has been addressed by others is to just accept it and use large pumps to keep the pressures low in the differential pumping section of the system, by using foils across the holes if the beam power is low enough, or to limit the pressure in the gas cell.
Beam powers used to date have been low enough that solid or rotating-disk beam stops have been acceptable. In addition, since users have thus far not been interested in high-resolution radiographs and images of objects, shadows produced by the beam stop were not important.
Most users to date are less concerned with having a purely monoenergetic neutron beam since they are doing “binary radiography”, i.e., they are looking to see if something is there or not, so stopping the beam on a solid is perfectly acceptable.
By exploring the approach of moving a neutral gas rapidly across the beam path by exploiting the pressure drop in a venturi, it appears that nominal gas density can be maintained in front of the beam since the region of rarefaction due to beam heating rapidly moves away from the beam focus channel carried by the momentum of the mass flow.
Others have tried to combat rarefaction by physically moving a large enclosed volume of gas rapidly in front of a particle beam. Another approach is to try and pre-cool the gas in the gas cell before the beam impinges, so the amount of time it takes for the density to drop is slightly increased, though this effect would improve the time-integrated neutron yield only slightly.
It is desirable to improve the sealing performance of the turbulent volume gas cell aperture system. It is also desirable to address problems associated with gas rarefaction and de-densification due to the particle beam impinging on and depositing energy in the neutral gas in the gas cell. It is further desirable to improve and extend the operating range of a “windowless” rotating aperture system.